1. Field of the Invention
The present invention relates to apparatus and method for tensile testing of specimens under applied and reduced loads and, more particularly, to apparatus and method for over-peak loading for creep-rupture testing.
2. Description of Related Art
Evaluating the condition and determining the future performance of mechanical components, such as gas turbine engine components, that operate in a high stress regime of materials is important to industry for maximizing the life of the components and limitations of remaining useful life, or life expended. Components which operate at high temperatures, such as greater than about 900. degree. F. (482. degree. C.), where a combination of creep and thermal aging of the components' material is of prime concern and require special consideration in order to achieve an acceptable remaining useful life estimation.
Many systems and methods for testing and estimating the useful life of such components involve applied mechanical loads that are cyclic and vary over a time period. Of particular interest is low cycle fatigue (LCF) testing, and especially sustained-peak LCF (SPLCF) testing, to examine the fatigue crack growth behavior over time of materials used to make gas turbine engine components. The fatigue crack growth behavior of specimens comprising these materials is characterized by applying cyclic loads using a “creep-rupture” frame. Various cyclic tensile amplitudes are applied, and the number of cycles required to pull apart the specimen under those conditions is recorded. Stress and/or fatigue damage is evidenced by a decrease in strength and stiffness. In some cases, the tests can be terminated after some number of cyclic loadings and then breaking the specimen (i.e., a tensile test) to determine the residual strength. The data from such destructive tests are usually characterized by empirical means and generalized by implication or extrapolation to a variety of service conditions for which the materials were not specifically tested in the laboratory.
Evaluation of test specimens for long hold-time fatigue and crack growth, as well as other stress-related properties, may be accomplished using a creep-rupture frame or lever arm tester such as the one disclosed in U.S. Pat. No. 5,345,826 (Strong), issued Sep. 13, 1994, which schematically illustrates a typical “creep-rupture” frame/lever arm tester. This device consists of a lever arm of from typically twelve to twenty inches in length that is pivotally mounted on a vertical frame at a point along the lever arm's length between its center and an end to which one end of a test specimen is attached. The other end of the test specimen is attached to a fixed base plate. When weights are applied or loaded on the opposite end of the lever arm, a tensile force is exerted on the test specimen according to the formula t=(wl)/d, where t is the tensile force exerted on the test specimen, w is the weight applied to the far end of the lever arm, l is the distance between the lever arm pivot point and the end carrying the applied weights, and d is the distance between the lever arm pivot point and the end connected to the test specimen. Tensile testing of the specimen is initiated by applying the force t.
Creep-rupture frames/lever arm testers can be equipped to cyclically apply and reduce the load (e.g., created by the weights) on the test specimen. The cyclical application and reduction of the load in creep-rupture frames/lever arm testers is conventionally carried out by using either a standard pneumatic cylinder or a scissor jack lift.
An apparatus and method for cyclical application and reduction of loads in tensile testing of specimens that provides a relatively smooth application and reduction of the applied loads is disclosed in U.S. Pat. No. 7,353,715 by Jeffrey Lynn Myers (the present inventor), issued Apr. 8, 2008, and assigned to the same assignee as the present patent application, the General Electric Company. U.S. Pat. No. 7,353,715 is incorporated herein by reference.
Another type of closely related tensile testing of specimens under applied and reduced loads is over-peak fatigue testing. The loading is typically performed on relatively expensive servo-hydraulic testing machines that are a great deal more expensive as is the testing as compared to creep-rupture testing machines and systems.
There exists a need for apparatus and methods for over-peak fatigue testing that are less expensive than those currently employed. There exists a need for apparatus and methods for over-peak fatigue testing that provides control of both over-peak load magnitude and dwell time. It is also desirable to provide a testing machine that evaluates creep-rupture and over-peak loading.